WASHINGTON — Confirming at least 40 cases of swine flu in the U.S., the Obama administration said Monday it was responding aggressively as if the outbreak would spread into a full pandemic. Officials urged Americans against most travel to Mexico as the virus that began there spread to the United States and beyond.

President Barack Obama urged calm, saying there was reason for concern but not yet "a cause for alarm."

Yet just in case, administration officials said that they were already waging a vigorous campaign of prevention, unsure of the outbreak's severity or where it would show up next.

U.S. customs officials began checking people entering U.S. territory. Millions of doses of flu-fighting medications from a federal stockpile were on their way to states, with priority given to the five already affected and to border states. Federal agencies were conferring with state and international governments.

"We want to make sure that we have equipment where it needs to be, people where they need to be and, most important, information shared at all levels," Janet Napolitano, head of the Homeland Security Department, told reporters.

Her briefing came shortly before the World Health Organization raised the severity of its pandemic alert level to four from three on a six-point scale. Level four means there is sustained human-to-human spread in at least one country. Level six is a full-fledged pandemic, an epidemic that has spread to a wide geographic area.

"We are proceeding as if we are preparatory to a full pandemic," Napolitano said.

She said travel warnings for trips to Mexico would remain in place as long as swine flu is detected.

Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said that so far the disease in the United States seemed less severe than the outbreak in Mexico, where more than 1,600 cases had been reported and where the suspected death toll had climbed to 149. No deaths had been reported in the U.S, and only one hospitalization.

"I wouldn't be overly reassured by that," Besser told reporters at CDC's headquarters in Atlanta. He raised the possibility of more severe cases — and deaths — in the United States.

A European Union official warned against travel to parts of the U.S. as well as Mexico, but Besser said that seemed unwarranted.

State Department spokesman Robert A. Wood said the EU commissioner's remarks were his "personal opinion," not an official position, and thus the department had no comment. "We don't want people to panic at this point," Wood said.